Gelation of poly(vinyl alcohol) solutions at low temperatures (20 to −78°C) and properties of gels

The flow points of atactic poly(vinyl alcohol) (a‐PVA) gels with H2O/dimethyl sulfoxide (DMSO) = 90/10 (v/v) chilled at 20 to −78°C for 24 h depended on the chilling temperature and were 0–30°C for gels with the initial polymer concentrations (Ci) of 2–5 g/dL, whereas those for H2O/DMSO = 50/50 chilled at 0 to −78°C were independent of the chilling temperature and were 70–75°C. Syneresis occurred after eight cycles of freezing (−24°C) and thawing (20°C) for a‐PVA hydrogels at concentrations above Ci = 4 g/dL and two such cycles for syndiotacticity‐rich PVA (s‐PVA) hydrogels at concentrations above Ci = 1 g/dL. The extent of syneresis per one cycle for s‐PVA hydrogels was higher than that for a‐PVA hydrogels at the initial cycles. In the a‐PVA hydrogels with an initial polymer concentration of ca. 30 g/dL, syneresis was expected not to occur even after 20 cycles. If all the free water in the gels is assumed to have transuded by syneresis after 20 cycles, the residual water is bound water and is estimated to be six water molecules per one vinyl alcohol monomer unit.